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Tim Folger's article on rare earths appeared last
June. His family hails from Nantucket, but he lives
now in Gallup, New Mexico, far from the sea.
changed history. Some archaeologists have ar-
gued, for instance, that a Mediterranean tsunami
struck the north shore of Crete a bit over 3,500
years ago; the disaster, they say, sent Minoan civi-
lization, one of the most sophisticated of the age,
into a tailspin, leading it to succumb to Myce-
naean Greeks. In 1755, when an earthquake and
tsunami killed tens of thousands in Lisbon, the
tragedy had a lasting impact on Western thought:
It helped demolish the complacent optimism of
the day. In Voltaire's novel Candide the blinkered
philosopher Pangloss arrives in Lisbon during
the catastrophe, persists in arguing that "all is for
the best in the best of all possible worlds," and
gets hanged for his trouble. Voltaire's withering
satire made it a little harder to be Panglossian---
to believe that a benevolent God designed an
optimal Earth.
In the h century . . the Greek historian
ucydides was the rst person to document the
connection between earthquakes and tsunamis.
He noticed that the rst sign of a tsunami is of-
ten the abrupt draining of a harbor, as the sea
pulls away from the coast. "Without an earth-
quake I do not see how such things could hap-
pen," he wrote. Actually they can. e Minoan
tsunami was triggered by the cataclysmic erup-
tion of ira, a volcanic island 70 miles north of
Crete in the Aegean. And landslides can cause
local tsunamis, such as the one that surged 1,700
feet up a hillside in Lituya Bay, Alaska, in 1958
(see page 70). All it takes is a large mass of rock
moving abruptly in a large mass of water---not
necessarily the ocean.
e vast majority of tsunamis, however, in-
cluding the Tohoku one, are caused by sea oor
earthquakes along faults called subduction
zones. Most are in the Paci c and Indian Oceans.
Along those boundaries two of Earth's tectonic
plates collide, and the one carrying dense oceanic
crust dives under the more buoyant continental
one, forming a deep-ocean trench. Normally this
happens smoothly, at a rate of a few inches a year.
But at some times and places the plates become
stuck---the peak of a subducting seamount
might snag on the bottom of a
continent, for example. After
centuries the accumulated strain
overwhelms the friction, and the
plates shudder past each other.
O Japan last March the quake
began miles below the sea oor
and then spread up the sloping
contact between the plates to
the Japan Trench at the sea oor. It released the
energy equivalent of 8,000 Hiroshima bombs.
A sizable fraction of that went into motion of
the sea oor, which raised and lowered the water
above it---thus creating a tsunami.
Ordinary ocean waves are mere wind-driven
wrinkles in the sea surface, but a tsunami moves
the entire water column, from the sea oor up.
e initial disturbance spreads out in opposite
directions from the fault, in long wave fronts
that may be a few hundred miles apart. In deep
water o shore they're barely noticeable. ey
grow to dangerous heights only in shallow water,
as they pile up against a coast---and they can
remain dangerous even a er they've crossed a
whole ocean, barreling at the speed of a jetliner.
The tsunami that savaged Japan last March
swept a man in California out to sea; it broke
Manhattan-size blocks of ice o the frozen mar-
gins of Antarctica. e tsunami that took 41 lives
in Minamisanriku in 1960 was triggered by a
magnitude 9.5 earthquake o Chile, the largest
quake on record.
The Indonesian tsunami of December 26,
2004, killed people all around the Indian Ocean.
It began o the northwest coast of Sumatra with
a sudden, thousand-mile-long rupture---and
Tsunamis remain dangerous after they've
crossed a whole ocean, barreling at the
speed of a jetliner. The one that savaged
Japan swept a man in California out to sea.